Several methods are known for measuring the sharpness of knives, for example, of the type used on slitters, choppers and other similar machines. Such machines are typically used in cutting webs, such as plastic sheet, film supports, cloth material and the like. One of the known methods is to take a plastic mold of the knife edge. When the mold has cured, it is removed from the knife and sliced with a razor. The knife profile is then viewed under a microscope to get a visual indication of the sharpness of the cutting edge of the knife. This requires that a specially trained technician make a subjective determination of the knife sharpness and the point at which the knife should be re-sharpened.
Another method is to use a small piece of lead on the end of a punch-like device. A technician places the lead directly over the cutting edge of the knife and strikes the punch, causing a small indentation in the lead corresponding to the knife profile. This lead model is then cut and the knife profile examined in a manner similar to the plastic mold method. In addition to the reliance on the subjective judgment of a trained technician, other problems with this method are that the results are dependent on the angle that the punch is held over the knife edge and the force with which the punch is struck. Additionally, there is the possibility that the knife edge can be damaged in the process.
Non-contact capacitance measurement techniques, in general, are also known. Examples are found in U.S. Pat. Nos. 3,641,431-Pigage et al and 4,620,281-Thompson et al. The '431 patent involves measurement of radial trueness of the position of cutter blades retained in an indexer of a gear cutting machine. A capacitance probe is mounted in a holder positioned adjacent the blades and measured capacitance of the air gap between the probe and the face or edge of the cutter blade is used to determine radial trueness of the blade position in the indexer. This arrangement does not measure sharpness of the cutting edge of the cutter blades, however. In the '281 patent, the condition of a cutting tool is sensed during the cutting operation by means of a capacitive sensor mounted on the cutting tool. The capacitive sensor is used to measure the distance between the tool sensor and the freshly cut surface of the workpiece being cut. Reduction in the measured distance provides an indication of nose wear of the cutting tool. Such an arrangement does not provide a direct measurement of knife sharpness. In addition, it assumes the ability to measure capacitance between the capacitive probe and a workpiece and, as such, is not suitable for use in directly measuring the sharpness of knives used in slitter and chopper equipment used for cutting webs and the like.
It is therefore an object of the present invention to provide a method and apparatus for measuring knife sharpness directly and in a non-contact manner.
It is a further object of the invention to provide non-contact knife sharpness measurement which is simple in structure and operation and provides repeatable measurement with a high degree of accuracy.
It is another object of the invention to provide non-contact knife sharpness measurement that directly measures the sharpness of the knife without relying on subjective judgment of a technician.